1. Field of the Invention
The present invention relates to a scanning optical lens having an asymmetric curvature and a laser scanning unit employing the same. More particularly, the present invention relates to a scanning optical lens having an asymmetric curvature in a sub-scanning direction, and a laser scanning unit for reducing a distance from a deflection unit to a photosensitive drum by employing the scanning optical lens.
2. Description of the Related Art
A Laser Scanning Unit (LSU) is an image forming apparatus for scanning and imaging a laser beam on a photosensitive drum to form an image in a printing apparatus such as a laser printer. FIG. 1 is a view schematically illustrating a conventional laser scanning unit. As shown in FIG. 1, the laser scanning unit is comprised of a laser source 11, a collimating lens 12, an aperture stop 13, a cylindrical lens 14, a beam deflection unit 15, a scanning optical lens 16, a photosensitive drum 18 and synchronous signal detecting units 19, 20, and 21. The laser emitting from the laser source 11 is converted into a parallel beam by the collimating lens 12, and the parallel beam is limited in shape by the aperture stop 13 and passes through the cylindrical lens 14. At this time, the parallel beam is converged in a sub-scanning direction by the cylindrical lens 14. After that, the converged laser is deflected in a main-scanning direction (that is, in a horizontal direction) by the beam deflection unit 15 rotating at a high speed while being imaged on the photosensitive drum 18 through the scanning optical lens 16 and deflector 17.
Here, the scanning optical lens 16, which is called an “F-theta lens”, functions to focus the laser on a surface of the photosensitive drum 18. In the apparatus shown in FIG. 1, the scanning optical lens 16 should be able to have a constant refractive index with respect to an optical axis and correct aberration. Further, the scanning optical lens 16 should also function to correct a deviation of the laser deflected at the beam deflection unit 15.
In order to perform this function, the scanning optical lens 16 has a specific configuration as shown in FIG. 2A, unlike a general lens. That is, as shown in FIG. 2B, a main-scanning direction section is convexed at an incidence surface and at an exit surface. The exit surface is a circular arc shape, whereas the incidence surface is a non-circular arc shape. As shown in FIG. 2C, in the sub-scanning direction section, an A-A′ section of a central portion of the lens 16 of FIG. 2A, has both convexed shapes, in which both the incidence surface and the exit surface are convexed. In contrast, as shown in FIG. 2D, a B-B′ section of an edge portion of the lens 16 of FIG. 2A, is concave-shaped at the incidence surface, and is convex-shaped at the exit surface.
As shown in the graphs of FIGS. 3A and 3B, a curvature of the incidence surface and the exit surface in the sub-scanning direction section is designed to continuously vary in the main-scanning direction. As appreciated from the graphs, in the incidence surface, the curvature is gradually decreased with reference to a center of the lens while being varied from positive (+) to negative (−) at the edge portion. That is, the center portion is convexed, but the edge portion is concaved as the curvature is gradually decreased. Meanwhile, in the exit surface, the curvature is gradually increased and again begins to reduce with reference to the center of the lens. However, since the curvature is always negative (−) at the exit surface, the exit surface is always convex-shaped. Accordingly, the scanning optical lens 16 is a kind of Anamophic lens having a different magnification in a vertical direction and in a horizontal direction.
Although the above conventional scanning optical lens 16 can beneficially correct optical aberration by using one lens, the lens 16 has a disadvantage in that an effective scanning angle cannot be increased to a great extent. Accordingly, since it is then impossible to shorten a distance from the beam deflection unit 15 to the photosensitive drum 18, there is a limit to the miniaturization of the conventional laser scanning unit. Further, since the scanning optical lens 16 has a relatively high image magnification in the sub-scanning direction, a stable optical performance is limited to that extent, and an improvement in the quality of a printed image is limited. This results since there is a performance non-stabilization factor and the laser beam is not uniform within an image forming area as the imaging magnification is enlarged.
Further, there is a disadvantage in that a multi-beam laser scanning unit, developed for providing faster printing, has the high imaging magnification in a sub-scanning direction. The multi-beam laser scanning unit scans a plurality of lasers in the sub-scanning direction at one time, and emits a plurality of lasers in the sub-scanning direction at the same time. In the case wherein the multi-beam laser scanning unit prints at an image quality of 600 dpi, a dot pitch in the sub-scanning direction should be about 42 μm on the photosensitive drum 18. However, the multi-beam laser scanning unit has a drawback in that in the case wherein the imaging magnification is more than five times as much as in the sub-scanning direction, an interval between the laser sources should be less than 10 μm in the sub-scanning direction, which is unrealizable.
Still further, the conventional scanning optical lens 16 is designed to have the curvature varying from positive (+) to negative (−), or from negative (−) to positive (+), at any point in the sub-scanning direction. This means that the lens 16 varies from a convex state (or concave state) to a plane state at any point and then, to a concave state (or convex state). However, in the case wherein a symbol of the curvature is changed, it is difficult to form a metal pattern for injecting the scanning optical lens. Accordingly, there is a drawback in that it is difficult to manufacture the scanning optical lens 16, and the resulting scanning optical lens is manufactured at a higher cost.
Accordingly, a need exists for a system and method for providing a laser scanning unit to enlarge the effective scanning angle and shorten the distance from the deflection unit to the photosensitive body, and reduce manufacturing costs while doing so.